High performance fired heater tubes

ABSTRACT

Fired heater tubes with improved resistance to corrosion and fouling suitable for use in thermal processing units of petroleum refineries and petrochemical plants are fabricated from a bulk alloy of an alumina-forming alloy which is capable of forming a stable oxide film on the surfaces of the tubes. The bulk alloy comprising about 3 wt. % to about 20 wt. % of Al, about 5 wt. % to about 30 wt. % Cr and at least one alloying component selected from Fe, Ni, Co, Si, Mn, B, C, N, P, Ga, Ge, As, In, Sn, Sb, Pb, Sc, La, Y, Ce, Ti, Zr, Hf, V, Nb, Ta, Mo, W, Re, Ru, Rh, Ir, Pd, Pt, Cu, Ag, Au and mixtures of these components.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent application Ser. No. 12/480,067, filed 8 Jun. 2009 which claimed priority from U.S. Provisional Patent Application No. 61/129,224, filed on Jun. 12, 2008. This application claims priority from U.S. applications Ser. Nos. 12/480,067 and 61/129,224.

FIELD OF THE INVENTION

This invention relates to the reduction of carburization and sulfidation corrosion and the reduction of depositional fouling in general and in particular to the reduction of carburization and sulfidation corrosion and the reduction of depositional fouling in fired heater tubes in refinery process units, petrochemical processing facilities, and in other ancillary and related industries such as synthetic fuels processes, (e.g., coal to liquids, coal gasification and gas to liquids) and in other components used for transporting or conveying process streams, which may be prone to corrosion and fouling. The present invention also relates to the reduction of corrosion and fouling associated with process streams, which include but are not limited to heavy crude oils and resid streams. More specifically, the present invention is directed to a high performance alloy for use fabricating fired heater tubes with improved resistance to corrosion and fouling in thermal processing units and a method of making the same.

BACKGROUND OF THE INVENTION

Petroleum refineries process mineral oil crudes as well as the so-called synthetic crudes derived from processing of bitumens, shale, tar sands or extra heavy oils and are also processed in refinery operations. In typical refinery processes, stored heavy crude oil is desalted to remove contaminants (e.g., sand, salts and water) as the first step in the refining process. The desalted crude is then fractionated first in the atmospheric distillation tower into combustion gas (furnace fuel gas) and other gaseous light ends, liquid products, and an atmospheric resid fraction which itself is then sent to the vacuum distillation tower, where light vacuum gas oil and heavy vacuum gas oil are extracted from the resid. The remaining tarry fluid left near the base of the vacuum tower, the vacuum residue, can either be (i) claimed as asphalt, or (ii) subject to further processing, such as coking. In various coking processes, the resid is heated to high temperatures of 850-950° F. (454-510° C.) such that light boiling products are thermally cracked off of the aromatic cores in the resid and are distilled overhead, leaving solid coke as the remainder.

The delayed coking process is a widely practiced commercial coking process. Typically, a resid feed is heated to coking temperature by flowing through a furnace and then allowed to undergo thermal cracking at this elevated temperature after flowing into the bottom of a high, cylindrical drum. The volatile products are removed to a fractionator and coke accumulates in the drum. Heavy liquid product from the fractionator may be recycled back to the furnace with fresh feed. When the drum fills up with coke, the feed is switched to a second drum. The coke is removed from the drum by cutting with high-pressure water to prepare the drum for the next coking cycle.

In Fluid Coking™, the resid is sprayed onto a hot, fluidized bed of coke particles in a vessel (i.e., the reactor). The volatile products are removed to a fractionator while the coke particles are removed from the bottom of the vessel and transferred to another vessel (the burner or heater), where the coke is partially burned with air to provide heat for the process. The coke then is recirculated back to the reactor. Since this process produces much more coke than is required for heating the process, fluid coke is withdrawn at the bottom of the reactor.

In FLEXICOKING™, a third vessel (i.e., the gasifier), is added to the Fluid Coking process. In the gasifier, coke is gasified with steam and air in net reducing conditions to produce a low BTU gas containing hydrogen, carbon monoxide, nitrogen, and hydrogen sulfide. The hydrogen sulfide is removed using adsorption. The remaining low BTU gas is burned as a clean fuel within the refinery and/or in a nearby power plant.

Visbreaking is a low conversion thermal process used originally to reduce the resid viscosity for heavy fuel oil applications. Today, it often uses a resid that exceeds minimum heavy fuel oil specifications and converts just enough to obtain 15-30% transportation boiling range liquids and still have the heavy product meet heavy fuel oil specifications. Since this process cannot readily accommodate coke formation, it is the coke induction period that may limit conversion, rather than heavy fuel oil specifications. A visbreaker reactor may be similar to a delayed coker with a furnace tube followed by a soaker drum. However, the drum is much smaller in volume to limit the residence time with the entire liquid product flowing through the drum. Alternatively, the entire visbreaker may be a long tube coiled within a furnace. Upsets cause coke to form and accumulate on visbreaker walls, which requires periodic decoking.

The tube furnace provides the heat for the delayed coking process and similar tube furnaces are used in other refinery thermal processing units. Typically in the tube furnace, there are two to four passes per furnace. The tubes are mounted horizontally on the side and held in place with alloy hangers. Multiple burners are arranged along the bottom of the radiant wall opposite from the tubes and are fired vertically upward. Tall furnaces are advantageous since the roof tubes are less likely to have flame impingement and overheating by both radiation and convection. Normally just the radiant section of the heater is used to heat the oil for a delayed coker. The upper convection section of the coker heater is used in some refineries to preheat the oil going to the fractionator or for other uses (e.g., steam generation).

The radiant section tubes in the fired heaters used in many refinery thermal processing units can experience fouling on the inside and/or outside of the tube. External tube fouling occurs when the heater is oil fired. During oil combustion solid particulate matter is formed containing carbon, sulfur and metals which are present in fuel oil. This particulate matter will over time collect on external tube surfaces. As well as external tube fouling, internal fouling is encountered on the interior surfaces of the furnace tubes which are in contact with the feed stream being heated in the furnace. Fired heaters that heat crude and reduced crude usually experience the highest level of internal fouling. With these fluids, the fouling occurs due to (i) the presence of solids in the fluid, (ii) thermal cracking forming high molecular weight compounds and (iii) in situ corrosion products. All these materials can end up sticking to the tube wall and forming “coke”. Liquids lighter than crude can also form internal deposits. For example, fired heaters heating liquid naphtha can experience internal tube fouling due to corrosion products and/or polymerization reactions forming long chain molecules which stick to the tube wall. Internal tube fouling usually has a large impact on heater operation and thermal efficiency.

These foulant deposits can result in an increase in the radiant tube metal temperature (TMT). As the fouling accumulates inside the heater tube, an insulation barrier between the heated metal and the process fluid is formed, resulting in an increased TMT. Although heaters in fouling service are designed to accommodate a specified TMT increase above the clean tube condition a tube rupture is possible as a result of high TMT (due to lessened metal strength) if fouling is allowed to occur without intervention, To avoid tube rupture, heaters with internal coke deposits can be operated at reduced rates (and hence reduced efficiency and productivity) such that metallurgical constraints are not exceeded on the tubes and tube rupture is avoided. When that limit is reached steps must be taken to remove the foulant. Often this means the heater must be shut down for cleaning. A secondary effect of internal fouling is increased pressure drop, which limits capacity and throughput. Heaters in fouling service are also designed to accommodate a specified increase in pressure drop. In most cases, the TMT limit is reached before the pressure drop limit. With good operational practices, most coker furnaces can be operational for 18 months before decoking of the tubes is needed. Depending on the tube metallurgy, when temperatures approach 1250° F. (677° C.) on the exterior skin thermocouple, the furnace must be steam spalled and/or steam-air decoked or cooled down and cleaned by hydraulic or mechanical pigging.

During normal use, the internal surfaces of the fired heater tubes are subject to carburization sulfidation, naphthenic acid corrosion and other forms of high temperature corrosion as a result of the prolonged exposure to the stream of heavy crude oil, resid and other petroleum fractions. Carburization is a form of high temperature degradation, which occurs when carbon from the environment diffuses into the metal, usually forming carbides in the matrix and along grain boundaries at temperatures generally in excess of 1000° F. (538° C.). Carburized material suffers an increase in hardness and often a substantial reduction in toughness, becoming embrittled to the point of exhibiting internal creep damage due to the increased volume of the carbides. Crude oils and hydrocarbon fractions which contain reactive sulfur are corrosive to carbon and low/medium alloy steels at temperatures above 500° F. (260° C.) and will cause sulfidation corrosion which forms iron sulfide. This sulfide scale that is formed is often referred to as sulfide induced fouling. Those which contain naphthenic acidic components are corrosive to carbon and low/medium alloy steels at temperatures above 400° F. (204° C.) and directly remove metal from the surface of the fired heater tube. Corrosion on the internal surfaces of the fired heater tubes creates an uneven surface that can enhance fouling because the various particles found in the petroleum stream may attach themselves to the roughened surface. It is also suggested that corroded surfaces may also provide a surface which is more amenable to foulant lay down.

While fouling and coke formation are endemic in thermal processing of both mineral oils and the synthetic crudes, the synthetic crudes present additional fouling problems, as these feedstocks are too heavy and contaminant laden for the typical refinery to process. The materials are often pre-treated at the production site and then shipped to refineries as synthetic crudes. These crudes may contain fine particulate siliceous inorganic matter, such as in the case of tar sands. Some may also contain reactive olefinic materials that are prone to forming polymeric foulant deposits within the fired heater tubes.

Currently, there are various surface modification techniques available for reducing corrosion and fouling in the fired heater tubes for refinery operations. Most of them are based on thin film coatings and include alonizing, hexamethyldisilazane (HMDS) and liquid phase silicate coatings. Alonizing is a diffusion alloying method and applied to the metal surface at elevated temperatures. As a result, about 100 μm thick, aluminum enriched layer forms on the metal surface. However, this coating, as characteristic of all such relatively thin coatings, reveals poor mechanical integrity and thermal stability due to presence of voids, defects and intermetallic brittle phases in the layer and has low reliability.

There is therefore a need to significantly reduce corrosion and fouling in the fired heater tubes in refinery and petrochemical processing operations that minimizes or does not encounter the drawbacks associated with the current techniques. The present invention provides a new way to achieve stable, durable surfaces to resist high temperature corrosion and fouling in fired heater tubes, in refinery process units, petrochemical processing facilities and other components used for transporting or conveying process streams, which may be prone to fouling.

SUMMARY OF THE INVENTION

The present invention is directed to an alumina-forming alloy which is capable of forming a stable oxide—preferably alumina—film on the surfaces of articles formed of the alloy. These alloys yield improved adhesion of the surface oxide film or layer, which enhances spalling resistance and this, in turn, improves the alloy integrity, stability and durability and reduces corrosion and fouling of fired heater tubes exposed to heavy crude oils, resids and other stream encountered in thermal processing units in petroleum refineries and petrochemical plants. The present alloys offer significant advantages relative to known alloy compositions for use in fabricating fired heater tubes in terms of improved resistance to corrosion and fouling on the metal surfaces exposed to streams in such process units. The advantageous properties and/or characteristics of the alloy compositions are based, at least in part, on the structure of the oxide film formed on the surface of the metal. These advantages include improved corrosion resistance, decreased fouling, decreased coke deposition, improved ease of surface oxide film formation prior to and in use as well as improved adhesion of in-situ formed surface oxide films and improved oxide film spalling resistance. In addition, increased coke spalling on fired heater tubes is also obtained. The advantageous properties and/or characteristics of the alumina-forming alloy compositions arise, at least in part, on the chemistry of the alloy, which include, inter alia, about 3 wt. % to about 20 wt. % of Al, about 5 wt. % to about 30 wt. % Cr, when exposed to crude oils and resid streams in refinery process units.

The present invention provides a fired heater tube that has improved resistance to corrosion and fouling. The fired heater tube is used to raise the temperature of a process fluid or stream (e.g., a crude oil based stream to be processed in a refinery or petrochemical facility). The fired heater tube may be located in the radiant section tube of the furnace but is not limited to use in the radiant section as it also may be used in the convection section as well as in other fired heaters which are prone to corrosion and fouling when heavy crude oils, resid streams and other refinery and petrochemical streams likely to induce corrosion or fouling or both are flowing through it at elevated temperatures.

In accordance with the present invention, the fired heater tubes are formed from an alumina-forming bulk alloy that is resistant to carburization, naphthenic acid corrosion, sulfidation, and other forms of high temperature corrosion and fouling. The use of an alumina-forming bulk alloy that is resistant to corrosion and fouling significantly mitigates carburization, naphtanic acid corrosion, sulfidation and other forms of high temperature corrosion and suppresses fouling, which produces numerous benefits including (i) an increase in heating efficiency, (ii) a reduction in the overall amount of energy needed to heat the crude oil, (iii) an increase in refinery throughput and (iv) a significant reduction in refinery downtime.

According to the present invention, the alumina-forming bulk alloy from which the tubes are fabricated comprises about 3 wt. % to about 20 wt. % of Al, about 5 wt. % to about 30 wt. % Cr and one or more alloying component(s) selected from Fe, Ni, Co, Si, Mn, B, C, N, P, Ga, Ge, As, In, Sn, Sb, Pb, Sc, La, Y, Ce, Ti, Zr, Hf, V, Nb, Ta, Mo, W, Re, Ru, Rh, Ir, Pd, Pt, Cu, Ag, Au.

The term “alumina-forming” in reference to the present alloys means that the alloy is capable of forming a coherent, adherent, protective surface film of an oxide of one or more of the elemental metals in the alloy, i.e. alumina, chromia, silica, mullite, spinels, or mixtures thereof from the elemental metals in the alloy, i.e. Al, Cr, Si, Fe, Ni, Mn etc. upon encountering the conditions used to generate the film or upon use of the film in a fired heater with a hydrocarbon fluid passing over the alloy. Alumina is a preferred oxide, and can be formed on the present alloys when heated to elevated temperatures in most process environments having sufficient oxygen activity to allow oxidation to participate in the high temperature corrosion reaction regardless of the predominant mode of corrosion. In fact, the present alloys rely on the oxidation reaction to develop a protective oxide (alumina) scale to resist corrosion attacks, such as sulfidation, carburization, and other deposit corrosion.

Fired heater tubes formed from the alloy have an inner surface and an outer surface with an oxide layer is formed on one or both of the surfaces; this oxide layer is substantially comprised of alumina, chromia, silica, mullite, spinels, or mixtures of them; it is preferably composed of alumina. In order to reduce fouling and corrosion from the fluid flowing in the tube, the oxide layer is formed on the inner surface of the tube which is in contact with the hydrocarbon feedstock flows; the oxide layer may also be formed on the exterior surface.

Surfaces of fired heater tubes which would benefit from the use of the alumina-forming bulk alloys of the instant invention include apparatus, reactor systems and units that are in contact with heavy crude oils and resid streams at any time during use and particularly in thermal processing of these and other refinery and petrochemical streams. These apparatus, reactor systems and units include, but are not limited to, atmospheric and vacuum distillation pipestills, cokers and visbreakers in refinery processing facilities and other components used for transporting or conveying process streams, which may be prone to corrosion and fouling.

The oxide layer on the surface(s) of the tube is a mono-layer or is made up of multiple layers and comprised of alumina, chromia, silica, mullite, spinels and mixtures of these and may contain some component oxides formed from the element(s) constituting an alumina-forming bulk alloy. A preferred oxide layer is substantially alumina. The alumina layer is preferably formed from the alumina-forming bulk alloy of the composition comprising about 3 wt. % to about 20 wt. % of Al, about 5 wt. % to about 30 wt. % Cr and at least one of the other alloying elements described above. The thickness of an oxide layer typically ranges from at least about 1 nm to about 100 μm, preferably from at least about 10 nm to about 50 μm, more preferably from at least about 100 nm to about 10 μm.

While the bulk alloy itself should be pore-free, the superficial oxide layer on the alumina-forming bulk alloy of the present invention should have a low porosity in order to ensure a continued resistance to corrosion and fouling when exposed to crude oils and resid streams in refinery process units. The oxide layer of alumina-forming bulk alloy preferably has a porosity less than about 3 vol. %, preferably less than about 2 vol. %, more preferably less than about 1 vol. %, and even more preferably less than 0.5 vol. %. Excessive porosity in the oxide layer of the alumina-forming bulk alloy serves as a pathway for gaseous molecules of heavy crude oils and resid streams in refinery process units to transfer gaseous molecules to the interior mass of the alloy. The transfer of gaseous molecules through the porous oxide layer triggers corrosion in the bulk alloy with deterioration of its mechanical strength. Thus, it is advantageous for the oxide layer of an alloy containing a minimal amount to have minimal porosity.

This oxide layer on the surface of the alumina-forming bulk alloy will form in-situ during use of the tubes of the alumina-forming bulk alloy when exposed to heavy crude oils and resid streams in refinery process units. These streams will normally possess sufficient oxygen activity to allow oxidation of the bulk metal to form the required corrosion-resisting, fouling-resisting oxide film. Alternatively, the oxide layer on the surface of the alumina-forming bulk alloy may be formed prior to use by exposing the coated material to controlled low oxygen partial pressure environments.

The oxide layer on the surface of the alumina-forming bulk alloy may also be formed on the surface by exposing the alumina-forming bulk alloy to controlled low oxygen partial pressure environments. The controlled low oxygen partial pressure environments are gaseous environments having thermodynamic oxygen partial pressures less than that of air. Non-limiting examples of a controlled low oxygen partial pressure environment include refinery steam, gaseous H₂O:H₂ mixture and a gaseous CO₂:CO mixture. The controlled low oxygen partial pressure environment may further contain other gases such as CH₄, NH₃, N₂, O₂, He, Ar and hydrocarbons and enable formation of the stable oxide layer of alumina, chromia, silica, mullite, spinels, and mixtures of these oxides. In this way, the protective oxide layer is formed prior to exposure of the bulk alloy to heavy crude oils and resid streams in refinery process units. The preferred temperature range of the controlled low oxygen partial pressure environment is from about 300° C. to about 1000° C., preferably from about 400° C. to about 1000° C. Typical exposure times range from about 1 hour to about 500 hours, preferably from about 1 hour to about 300 hours, and more preferably from about 1 hour to about 100 hours.

The bright annealing process is a suitable procedure for forming the oxide film. Bright annealing is an annealing process that is carried out in a controlled atmosphere furnace or vacuum in order that oxidation is reduced to a minimum and the metal surface remains relatively bright. The process conditions such as atmosphere, temperature, time and heating/cooling rate utilized during the bright annealing process will be dependent on the metallurgy of the alloy being acted upon. As a non-limiting example, austenitic stainless steel can typically be bright annealed in either pure hydrogen or dissociated ammonia, provided that the dew point of the atmosphere is less than −50° C. and the tubes, upon entering the furnace, are dry and scrupulously clean. Bright annealing temperatures usually are above 1040° C. Time at temperature is often kept short to hold surface scaling to a minimum or to control grain growth.

The alumina-forming bulk alloy compositions of the present invention offer significant advantages relative to known alloy compositions for use as protective coatings in comparable applications. As a non-limiting example, alloying elements such as Al, Si, Sc, La, Y and Ce, provide improved adhesion of in-situ formed surface oxide films, which contributes to enhanced spalling resistance. These elements can be present in the metal oxide surface layer in the form of oxide particles. Non-limiting examples are Y₂O₃ and CeO₂. The alumina-forming bulk alloy may itself contain oxide particles in the form of an oxide dispersion strengthened (ODS) alloy.

Alloying elements such as Ga, Ge, As, In, Sn, Sb, Pb, Pd, Pt, Cu, Ag and Au, provide reduced fouling because these elements are non-catalytic to surface carbon transfer reaction. Alloying elements such as Re, Ru, Rh, Ir, Pd, Pt, Cu, Ag and Au provide increased coating integrity, stability and durability and provide a defect-free alumina layer formed from alloy compositions comprising about 3 wt. % to about 20 wt. % of Al, about 5 wt. % to about 30 wt. % Cr based on the total weight of the alumina-forming bulk alloy when these additional alloying components are present.

One group of the alumina-forming bulk alloys according to the invention is based on iron and comprises, in addition to iron as the balance, about 18 wt. % to about 30 wt. % Ni, about 12 wt. % to about 18 wt. % chromium, about 3.0 wt. % to about 5.5 wt. % aluminum, about 1.0 wt. % to about 3.0 wt. % of niobium, and about 0.01 wt. % to about 3.0 wt. % of at least one element selected from Mo, Mn, Si, Cu, W, V, Ti, Hf, Y, C, B, and P. Another group is also based on iron and comprises in addition to iron as the balance, about 20 wt. % to about 25 wt. % Ni, about 14 wt. % to about 16 wt. % Cr, about 3.0 wt. % to about 4.0 wt. % Al, about 1.0 wt. % to about 3.0 wt. % of niobium, and about 0.01 wt. % to about 3.0 wt. % of at least one element selected from Mo, Mn, Si, Cu, W, V, Ti, Hf, Y, C, B, and P.

A preferred group of alumina-forming bulk alloys which is based on iron comprises, in addition to iron as the balance, about 18 wt. % to about 30 wt. % nickel and more preferably about 20 wt. % to about 25 wt. % Ni. The presence of nickel in the alloy ensures austenitic structure of the alloy, which is preferred to provide superior mechanical properties at high temperatures. The alumina-forming bulk alloys of this type typically comprise about 12 wt. % to about 18 wt. % chromium, and preferably about 14 wt. % to about 16 wt. % Cr with about 3.0 wt. % to about 5.5 wt. % aluminum, preferably about 3.0 wt. % to about 4.0 wt. % Al. The presence of both Cr and Al in the alloy ensures formation of the desired protective alumina scale on the interior and exterior surface of tubes fabricated from the alloy. The alloy also preferably comprises about 1.0 wt. % to about 3.0 wt. % of niobium; the presence of this component in the alloy ensures formation of Nb-containing, MC-type carbide precipitates in the interior of the alloy. Such carbide precipitates promote creep resistance at high temperatures. The alumina-forming bulk alloy further and preferably comprises about 0.01 wt. % to about 3.0 wt. % of at least one element selected from Mo, Mn, Si, Cu, W, V, Ti, Hf, Y, C, B, and P. Similar to Nb, strong carbide forming elements such as Mo, W, V and Ti also contribute formation of nano-scale MC-type carbide precipitates. It is also preferred that the bulk alloy comprises less than about 0.2 wt. % silicon, and preferably less than about 0.15 wt. % silicon. Excessive amounts of silicon in the alumina-forming bulk alloy may induce solidification-induced cracking during fabrication process. It is also preferred that the bulk alloy comprises less than about 0.15 wt. % carbon, and preferably less than about 0.1 wt. % carbon. Carbon can be present in the alumina-forming bulk alloy as a form of carbide precipitates, which provide creep strength of the alumina-forming bulk alloy when it is exposed to high temperatures for extended period of time.

As a non-liming example, a preferred alumina-forming bulk alloy is based on iron and comprises about 25.0 wt. % Ni, 15.0 wt. % Cr, 4.0 wt. % Al, 2.5 wt. % Nb, 2.0 wt. % Mo, 2.0 wt. % Mn, 0.15 wt. % Si, 0.1 wt. % C, 0.01 wt. % B, 0.15 wt. % Hf, 0.07 wt. % Y balance Fe. Another non-limiting example, the alumina-forming bulk alloy is based on iron and comprises about 25.0 wt. % Ni, 14.0 wt. % Cr, 3.55 wt. % Al, 2.53 wt. % Nb, 2.0 wt. % Mo, 2.0 wt. % Mn, 0.14 wt. % Si, 0.1 wt. % C, 0.51 wt. % Cu, 0.95 wt. % W, 0.04 wt. % V, 0.05 wt. % Ti, balance Fe.

A preferred group of alumina-forming alloys suitable for fired heater tube fabrication comprises the iron-based alloys with 3 wt. % to about 20 wt. % of Al, about 5 wt. % to about 30 wt. % Cr and at least one of the specified alloying components described above. Specific examples of such alloys are set out in Table 1 below. The alumina-forming austenitic (AFA) stainless steels developed by Oak Ridge National Laboratory (ORNL) are especially preferred with specific examples noted in Table 1 below. The AFA stainless steels are unique in that the composition allows for alumina scales to form on the exterior surface of the steel, which provides significant oxidation resistance, and displays excellent creep strength at high temperatures (700-800° C.).

TABLE 1 Alloy Names Compositions (wt. %) AFA-17 Bal.Fe: 15.0 Cr: 3.0 Al: 25.0 Ni: 2.5 Nb: 2.0 Mo: 2.0 Mn: 0.15 Si: 0.1 C: 0.01 B: 0.15 Hf: 0.07Y AFA- 20 Bal.Fe: 15.0 Cr: 4.0 Al: 25.0 Ni: 2.5 Nb: 2.0 Mo: 2.0 Mn: 0.15 Si: 0.1 C: 0.01 B: 0.15 Hf: 0.07Y AFA- OC1 Bal.Fe: 14.2 Cr: 3.0 Al: 20.0 Ni: 2.5 Nb: 2.0 Mo: 2.0 Mn: 0.14 Si: 0.1 C: 0.02 P: 0.51 Cu: 0.97 W: 0.04 V: 0.05Ti AFA- OC2 Bal.Fe: 14.3 Cr: 3.0 Al: 25.0 Ni: 1.0 Nb: 2.0 Mo: 2.0 Mn: 0.14 Si: 0.05 C: 0.51 Cu: 0.96 W: 0.04 V: 0.05Ti AFA- OC4 Bal.Fe: 14.0 Cr: 3.55 Al: 25.0 Ni: 2.53 Nb: 2.0 Mo: 2.0 Mn: 0.14 Si: 0.1 C: 0.51 Cu: 0.95 W: 0.04 V: 0.05Ti

Regardless of alloy composition, the interior surface of the wall of the fired heater tubes is preferably formed to have an average surface roughness (Ra) of less than 40 micro inches (1.1 μm). Preferably, the surface roughness is less than 20 micro inches (0.5 μm). More preferably, the surface roughness is less than 10 micro inches (0.25 μm). The inner surfaces of the plurality of the fired heater tubes are preferred to have the above-mentioned surface roughness to further reduce fouling from heavy oil and resid streams flowing through the tubes. Roughness is routinely expressed as the arithmetic average roughness (Ra). The arithmetic average height of roughness component of irregularities from the mean line is measured within the sample length L. The standard cut-off is 0.8 mm with a measuring length of 4.8 mm. This measurement employed in determining the surface roughness in accordance with the present invention conforms to ANSI/ASME B46.1 “Surface Texture- Surface Roughness, Waviness and Lay”.

The surface roughness may be reduced in several ways including mechanical polishing, electro polishing and lapping. There are additional benefits of reducing the surface roughness of the coated metal. One of the benefits is the shifting from a linear growth rate of the foulant, which results in the continuous thickening of the foulant deposit; to an asymptotic growth rate which reaches a finite thickness and then stops thickening.

The following examples illustrate the present invention and its advantages.

For the commercially available alloys listed in Table 1, square samples of 10 mm×10 mm×1.5 mm were prepared from alloy sheets. All the samples were exposed to heavy crude resid at 1000°˜1056° F. (538°˜560° C.) for 4˜20 hours in a tubing bomb test apparatus. After testing, the specimens were cleaned in toluene and acetone sequentially and characterized by selected analytical instruments. Both surface and cross sectional images of the tested specimen were examined using Scanning Electron Microscopy (SEM). The atomic percent of elements in the oxide layer and the coating metal is determined by standard Auger Electron Spectroscopy (AES) analysis. A focused electron beam irradiates a specimen surface and produces Auger electrons, whose energies are characteristic of the element from which they are generated. Compositional depth profiling of elements is accomplished by using an independent ion beam to sputter the sample surface while using AES to analyze each successive depth.

EXAMPLE 1 Comparative

Following the test methods described above, a mechanically polished Kanthal APM sample was tested. Kanthal APM is an advanced powder metallurgical, dispersion strengthened, ferritic iron-chromium-aluminum alloy (FeCrAl alloy) for use at tube temperatures up to 1250° C. Kanthal APM tubes have good form stability at high temperature. Kanthal APM forms an excellent, non-scaling surface alumina film, which gives good protection in most furnace environments, i.e. oxidizing, sulfidizing and carburizing, as well as against deposits of carbon, ash, etc. Kanthal APM contains 5.8 wt. % Al, 20.5-23.5 wt. % Cr, 0.5 wt. % Mn, 0.7 wt. % Si, and 0.08 wt. % C with a balance being Fe.

Surface and cross-sectional SEM images of the corrosion surface of a mechanically polished Kanthal APM were obtained after reaction at 1000° F. (538° C.) in heavy resid-content crude for 4 hours. No significant corrosion or fouling deposits were observed after the specimen was cleaned in toluene and acetone sequentially. AES concentration depth profile of the corrosion surface of the same sample was examined. The carbon peak found near the surface was probably caused by remnants of crude deposits. Also identified was about 200 nm thick corrosion scale, which was mainly comprised of Cr—Fe sulfide and Cr—Al oxide. Under this layer, about 200 nm thick alumina sublayer formation was observed. This alumina layer provides superior corrosion resistance of the alumina forming alloy, which is prerequisite for fouling mitigation.

EXAMPLE 2 Comparative

Following the test methods described above, a 120 grit finished Kanthal APM sample was tested. Surface and cross-sectional SEM images of the corrosion surface of a 120 grit finished Kanthal APM were obtained after reaction at 1000° F. (538° C.) in heavy resid-content crude for 4 hours. After cleaned the specimen in in toluene and acetone sequentially, no significant corrosion scale was observed. However, some thin layer of carbon deposit was observed on the surface, whose deposit appeared to be anchored to the roughened surface of the metal. Superior corrosion resistance was attributed to alumina layer formed on the metal surface. The thickness of alumina layer was about 200 nm measured by AES.

The effect of surface roughness in reduction of carbon deposit was examined by electron microscopic investigation. Two samples were tested and cleaned at the same experimental conditions. The thickness of the carbon deposit on the rough surface (e.g. 120 grit finish) was about 4 microns and uniformly present on the surface. The average surface roughness (Ra) of the 120 grit finish surface measured by a skidded contact profilometer was about 80 micro inches (2.2 μm). By contrast, no carbon deposit was found on the smooth surface (e.g. mechanically polished). The average surface roughness (Ra) of the mechanically polished surface measured by a skidded contact profilometer was about 40 micro inches (1.1 μm). The metal surface having the reduced surface roughness exhibited less fouling. Both surfaces revealed good corrosion resistance as confirmed by a protective alumina layer formed in-situ during testing. The reduction in carbon deposit on a smooth surface illustrates the additional benefit of the surface smoothness.

EXAMPLE 3 Comparative

Following the test methods described above, a 120 grit finished 304L SS sample was tested. Surface and cross-sectional SEM images of the corrosion surface of a 120 grit finished 304L SS were obtained after reaction at 1000° F. (538° C.) in heavy resid-content crude for 4 hours. Formation of thick (about 8 μm) multi-layered corrosion scale was observed. Corrosion scales were comprised of Fe sulfide, Fe—Cr sulfide, thiospinel and Fe—Cr oxysulfide based on Energy Dispersive X-ray Spectroscopy (EDXS) characterization. In comparison with the same surface finished Kanthal APM (Example 2), the thickness of corrosion scale on 304L SS was about 40 times thicker (8000 nm vs. 200 nm). This result clearly confirms that the alumina layer formed on Kanthal APM surface is much more resistant to corrosion than the corrosion scales formed on 304L SS surface.

EXAMPLE 4 AFA-20

An alumina forming alloy (AFA-20 in Table 1, Bal.Fe:15.0Cr:4.0Al:25.0Ni:2.5Nb:2.0Mo:2.0Mn:0.15Si:0.1C:0.01B:0.15Hf:0.07Y) was prepared by arc melting and solidification. A square specimen of 10 mm×10 mm×1.5 mm was used for a lab reactor test. Following the test method described above, the AFA-20 specimen was tested at 1056° F. (560° C.) for 20 hours in a heavy resid medium. No corrosion was observed on the AFA-20 metal surface after the specimen was retrieved from a reactor. A protective oxide (alumina) layer about 70 nm thick was identified on the surface of the alumina-forming alloy, providing superior corrosion resistance.

EXAMPLE 5 AFA-OC4

An alumina-forming alloy (AFA-OC4 in Table 1, Bal.Fe:14.0Cr:3.55Al:25.0Ni:2.53Nb:2.0Mo:2.0Mn:0.14Si:0.1C:0.51Cu:0.95W:0.04V:0.05Ti) was prepared by arc melting and solidification. A square specimen of 10 mm×10 mm×1.5 mm was used for a lab reactor test. Following the test method described above, the AFA-OC4 specimen was tested at 1056° F. (560° C.) for 20 hours in a heavy resid medium. No corrosion was observed on the AFA-OC4 metal surface after the specimen was retrieved from a reactor. A protective oxide (alumina) layer about 70 nm thick was identified on the surface of the alumina forming alloy providing superior corrosion resistance. 

We claim:
 1. A fired heater tube having an inner surface and an outer surface formed from an alumina-forming bulk alloy comprising about 3 wt. % to about 20 wt. % of Al, about 5 wt. % to about 30 wt. % Cr and at least one element selected from the group consisting of Fe, Ni, Co, Si, Mn, B, C, N, P, Ga, Ge, As, In, Sn, Sb, Pb, Sc, La, Y, Ce, Ti, Zr, Hf, V, Nb, Ta, Mo, W, Re, Ru, Rh, Ir, Pd, Pt, Cu, Ag, Au and mixtures thereof, the tube having a surface oxide layer on at least one of its alumina-forming bulk alloy surfaces, substantially comprised of alumina, chromia, silica, mullite, spinels, or mixtures thereof.
 2. The fired heater tube of claim 1 in which the surface oxide layer is formed on the inner surface of the tube.
 3. The fired heater tube of claim 1 in which the oxide layer is alumina.
 4. The fired heater tube of claim 1 in which the alumina forming bulk alloy comprises about 3 wt. % to about 10 wt. % of aluminum.
 5. The fired heater tube of claim 1 in which the alumina forming bulk alloy comprises about 3 wt. % to about 3.5 wt. % of aluminum.
 6. The fired heater tube of claim 5 in which the alumina forming bulk alloy comprises about 1 wt. % to about 3 wt. % of niobium.
 7. The fired heater tube of claim 1 in which the alumina forming bulk alloy comprises about 10 wt. % to about 20 wt. % of chromium.
 8. The fired heater tube of claim 1 in which the alumina forming bulk alloy comprises about 12 wt. % to about 18 wt. % of chromium.
 9. The fired heater tube of claim 1 in which the alumina forming bulk alloy comprises about 18 wt. % to about 30 wt. % of nickel.
 10. The fired heater tube of claim 1 in which the alumina forming bulk alloy comprises about 20 wt. % to about 25 wt. % of nickel.
 11. The fired heater tube of claim 1 in which the alumina forming bulk alloy comprises less than about 0.8 wt. % of silicon.
 12. The fired heater tube of claim 1 in which the alumina forming bulk alloy comprises less than about 0.15 wt. % carbon,
 13. The fired heater tube of claim 1 in which the alumina forming bulk alloy comprises less than about 0.1 wt. % carbon
 14. The fired heater tube according to claim 1 in which the alumina forming bulk alloy comprises about 0.01 wt. % to about 4.0 wt. % of at least one element selected from Mn, Ti, Zr, Hf, V, Nb, Ta, Mo and W.
 15. The fired heater tube according to claim 1 in which the alumina forming bulk alloy comprises about 0.01 wt. % to about 2.0 wt. % of at least one element selected from Al, Si, Sc, La, Y and Ce.
 16. The fired heater tube according to claim 1 in which the alumina forming bulk alloy comprises about 0.01 wt. % to about 2.0 wt. % of at least one element selected from Ga, Ge, As, In, Sn, Sb, Pb, Pd, Pt, Cu, Ag and Au.
 17. The fired heater tube according to claim 1 in which the alumina forming bulk alloy comprises about 0.01 wt. % to about 2.0 wt. % of at least one element selected from Re, Ru, Rh, Ir, Pd, Pt, Cu, Ag and Au.
 18. The fired heater tube according to claim 1 in which the alumina forming bulk alloy comprises about 0.01 wt. % to about 2.0 wt. % of oxide particles of at least one element selected from Al, Si, Sc, La, Y and Ce.
 19. The fired heater tube according to claim 1 in which the alumina forming bulk alloy comprises, in addition to iron as the balance, about 18 wt. % to about 30 wt. % Ni, about 12 wt. % to about 18 wt. % chromium, about 3.0 wt. % to about 5.5 wt. % aluminum, about 1.0 wt. % to about 3.0 wt. % of niobium, and about 0.01 wt. % to about 3.0 wt. % of at least one element selected from Mo, Mn, Si, Cu, W, V, Ti, Hf, Y, C, B, and P.
 20. The fired heater tube according to claim 1 in which the alumina forming bulk alloy comprises, in addition to iron as the balance, about 20 wt. % to about 25 wt. % Ni, about 14 wt. % to about 16 wt. % Cr, about 3.0 wt. % to about 4.0 wt. % Al, about 1.0 wt. % to about 3.0 wt. % of niobium, and about 0.01 wt. % to about 3.0 wt. % of at least one element selected from Mo, Mn, Si, Cu, W, V, Ti, Hf, Y, C, B, and P.
 21. The fired heater tube according to claim 1 in which the alumina forming bulk alloy comprises about 25.0 wt. % Ni, 15.0 wt. % Cr, 4.0 wt. % Al, 2.5 wt. % Nb, 2.0 wt. % Mo, 2.0 wt. % Mn, 0.15 wt. % Si, 0.1 wt. % C, 0.01 wt. % B, 0.15 wt. % Hf, 0.07 wt. % Y, balance Fe.
 22. The fired heater tube according to claim 1 in which the alumina forming bulk alloy comprises about 25.0 wt. % Ni, 14.0 wt. % Cr, 3.55 wt. % Al, 2.53 wt. % Nb, 2.0 wt. % Mo, 2.0 wt. % Mn, 0.14 wt. % Si, 0.1 wt. % C, 0.51 wt. % Cu, 0.95 wt. % W, 0.04 wt. % V, 0.05 wt. % Ti, balance Fe.
 21. The fired heater tube of claim 1 in which the oxide layer of the alumina forming bulk alloy has a porosity of less than about 3 volume percent.
 22. The fired heater tube of claim 1 in which the oxide layer of the alumina forming bulk alloy has a porosity of less than about 1 volume percent.
 23. The fired heater tube of claim 1 in which the oxide layer has a thickness from about 1 nm to about 100 μm.
 24. The fired heater tube of claim 1 in which the oxide layer has a thickness from 100 nm to about 10 μm.
 25. The fired heater tube according to claim 1 in which the coating metal layer has an average surface roughness (Ra) of less than 40 micro inches (1.1 μm).
 26. The fired heater tube according to claim 1 in which the coating metal layer has an average surface roughness (Ra) of less than 20 micro inches (0.5 μm)
 27. The fired heater tube according to claim 1 in which the alumina forming bulk alloy comprises about 15.0 wt. % Cr, about 3.0 wt. % Al, about 25.0 wt. % Ni, about 2.5 wt. % Nb, about 2.0 wt. % Mo, about 2.0 wt. % Mn, about 0.15 wt. % Si, about 0.1 wt. % C, about 0.01 wt. % B, about 0.15 wt. % Hf, about 0.07 wt. % Y, balance Fe.
 28. The fired heater tube according to claim 1 in which the alumina forming bulk alloy comprises about 15.0 wt. % Cr, about 4.0 wt. % Al, about 25.0 wt. % Ni, about 2.5 wt. % Nb, about 2.0 wt. % Mo, about 2.0 wt. % Mn, about 0.15 wt. % Si, about 0.1 wt. % C, about 0.01 wt. % B, about 0.15 wt. % Hf, about 0.07 wt. % Y, balance Fe.
 29. The fired heater tube according to claim 1 in which the alumina forming bulk alloy comprises about 14.2 wt. % Cr, about 3.0 wt. % Al, about 20.0 wt. % Ni, about 2.5 wt. % Nb, about 2.0 wt. % Mo, about 2.0 wt. % Mn, about 0.14 wt. % Si, about 0.1 wt. % C, about 0.02 wt. % P, about 0.51 wt. % Cu, about 0.97 wt. % W, about 0.04 wt. % V, about 0.05 wt. % Ti, balance Fe.
 30. The fired heater tube according to claim 1 in which the alumina forming bulk alloy comprises about 14.3 wt. % Cr, about wt. % 3.0Al, about 25.0 wt. % Ni, about 1.0 wt. % Nb, about 2.0 wt. % Mo, about 2.0 wt. % Mn, about 0.14 wt. % Si, about 0.05 wt. % C, about 0.51 wt. % Cu, about 0.96 wt. % W, about 0.04 wt. % V, about 0.05 wt. % Ti , balance Fe.
 31. The fired heater tube according to claim 1 in which the alumina forming bulk alloy comprises about 14.0 wt. % Cr, about 3.55 wt. % Al, about 25.0 wt. % Ni, about 2.53 wt. % Nb, about 2.0 wt. % Mo, about 2.0 wt. % Mn, about 0.14 wt. % Si, about 0.1wt. % C, about 0.51 wt. % Cu, about 0.95 wt. % W, about 0.04 wt. % V, about 0.05 wt. % Ti , balance Fe.32. A petroleum refinery thermal processing unit having a fired heater with heater tubes according to claim
 1. 33. A petroleum refinery thermal processing unit having a fired heater with heater tubes according to claim 1 in the radiant section.
 34. A petrochemical thermal processing unit having a fired heater with heater tubes according to claim
 1. 35. A petrochemical thermal processing unit having a fired heater with heater tubes according to claim 1 in the radiant section. 